May 2015 - Haskell-Cafe

Extensible states
by Alberto G. Corona 05 Jul '15

05 Jul '15

Hi, Anyone used some of the extensible record packages to create a kind of extensible state monad? I mean something that besides having "get", "gets" and "put" would have some kind of "add" and "gets": add :: a -> State () gets :: State (Maybe a) or add :: LabelName -> a -> State () gets :: LabelName -> State (Maybe a) So that I can extend the state without using additional monad transformers. Monad transformers are very hard for beginners and scramble error messages I did the first option for MFlow, hplayground and Transient packages (setSData and getSData). But my solution uses a map indexed by type and this requires a lookup for each access. I would like to know if there is an alternative with no lookups. I´m not obsessed with speed but In some applications the speed may be important.... Anyone? -- Alberto.

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MVars and locks
by Tom Ellis 04 Jul '15

04 Jul '15

In the following section of the book "Parallel and Concurrent Programming in Haskell" http://chimera.labs.oreilly.com/books/1230000000929/ch07.html#sec_conc-phon… Simon Marlow explains that MVars can be used to implement something like locks on shared functional state: "To acquire the lock, we take the MVar, whereas, to update the variable and release the lock, we put the MVar." Am I right in thinking this only holds if we are careful to ensure both those operations happen in the given order? For example, if I take the MVar (lock) and I am about to put something back in (unlock) but someone else puts something in before I do, then the lock system becomes broken, doesn't it? So if we want to be sure we have a robust lock system we have to wrap up MVars in another abstraction? Tom

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pqueue - repository and maintainer (taking over)
by lennart spitzner 24 Jun '15

24 Jun '15

Greetings, Regarding the pqueue package: 1) Does anyone have any (additional) contact details for the current maintainer/author, Louis Wasserman? 2) Is the package's repository available anywhere? The darcs link listed in the package 404s. 3) I would be willing to take over for basic maintenance if the current maintainer is unavailable. The pqueue package does not compile with ghc-7.10. Fixing requires just some quick changes (i have a working version locally). The last upload for the package is from 2012, but the author's github account lowasser shows some more recent activity. Yet I have not received a reply to my e-mail from six weeks ago. Lennart

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RFC: "Native -XCPP" Proposal
by Herbert Valerio Riedel 18 Jun '15

18 Jun '15

Hello *, As you may be aware, GHC's `{-# LANGUAGE CPP #-}` language extension currently relies on the system's C-compiler bundled `cpp` program to provide a "traditional mode" c-preprocessor. This has caused several problems in the past, since parsing Haskell code with a preprocessor mode designed for use with C's tokenizer has caused already quite some problems[1] in the past. I'd like to see GHC 7.12 adopt an implemntation of `-XCPP` that does not rely on the shaky system-`cpp` foundation. To this end I've created a wiki page https://ghc.haskell.org/trac/ghc/wiki/Proposal/NativeCpp to describe the actual problems in more detail, and a couple of possible ways forward. Ideally, we'd simply integrate `cpphs` into GHC (i.e. "plan 2"). However, due to `cpp`s non-BSD3 license this should be discussed and debated since affects the overall-license of the GHC code-base, which may or may not be a problem to GHC's user-base (and that's what I hope this discussion will help to find out). So please go ahead and read the Wiki page... and then speak your mind! Thanks, HVR [1]: ...does anybody remember the issues Haskell packages (& GHC) encountered when Apple switched to the Clang tool-chain, thereby causing code using `-XCPP` to suddenly break due to subtly different `cpp`-semantics?

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Recognizing projects schemes
by Ruben Astudillo 14 Jun '15

14 Jun '15

I recently came across the following blog-post: http://blog.cleancoder.com/uncle-bob/2015/04/15/DoesOrganizationMatter.html It speaks a bit of simplicity, efficiency and stuff that isn't important. What is important at least to me was the concept of project scheme, summarized on the following phrase: ``And so this gets to the crux of the question that you were really asking. You were asking whether the time required to learn the organization scheme of the system is worth the bother. Learning that organization scheme is hard. Becoming proficient at reading and changing the code within that scheme take time, effort, and practice. And that can feel like a waste when you compare it to how simple life was when you only had 100 lines of code.'' And that is something I totally struggle at approaching new projects. The only reason I could understand XMonad for example is because they gave a general overview (thanks) of it on the Developing module. I feel I got a problem of methodology. What approaches are you guys using to understanding new projects schemes on a efficient manner? How long it usually takes you?. Any advices? Thanks in advance. -- Ruben Astudillo. pgp: 0x3C332311 , usala en lo posible :-) Crear un haiku, en diecisiete silabas, es complica...

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ANN: nonce package
by Felipe Lessa 07 Jun '15

07 Jun '15

(Please forgive me if you received multiple copies of this e-mail.) Hello, The nonce package [1] contains functions to easily generate cryptographic nonces for many situations. Some places where these generated nonces can be used include: - Password recovery e-mail tokens. - XSRF protection tokens. - Session IDs sent on cookies. - Initialization vectors. It uses an AES CPRNG periodically reseeded from /dev/urandom (or equivalent). It has no frills, no knobs, so it's hard to misuse. It's been available for an year but I just realized I've never properly announced it. Regrettably, I've seen many uses of the random package (System.Random) when generating nonces. It's a bad choice: it is not a cryptographically secure PRNG, contains low entropy (64-bit state), and its default usage is seeded predictably (using a constant seed). Please avoid using the random package for generating nonces at all costs. In its stead, use the nonce package or something similar. Cheers, [1] http://hackage.haskell.org/package/nonce -- Felipe.

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Consecutive FFI calls
by David Turner 03 Jun '15

03 Jun '15

Hi, If I make a sequence of FFI calls (on a single Haskell thread) but which end up being called from different OS threads, is there any kind of ordering guarantee given? More specifically, is there a full memory barrier at the point where a Haskell thread migrates to a new OS thread? Many thanks, David

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Blog platform written in Haskell
by Nicola Gigante 02 Jun '15

02 Jun '15

Hi all, Is there a blog platform written in Haskell? I mean something dynamic (as opposed to a static site generator like hakyll) like WordPress but written in Haskell. Does something like that exist? Greetings, Nicola

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.a files not found
by Henk-Jan van Tuyl 30 May '15

30 May '15

L.S., I get messages about .a files not being found, when I try to install package data-default-instances-base, data-default-instances-containers, data-default-instances-dlist or data-default-instances-old-locale. How can I solve this? I am using GHC 7.10.1 on Windows. Sample output: C:\X\data-default-instances-old-locale-0.0.1> cabal install Resolving dependencies... Configuring data-default-instances-old-locale-0.0.1... Building data-default-instances-old-locale-0.0.1... Failed to install data-default-instances-old-locale-0.0.1 Build log ( C:\Users\X\AppData\Roaming\cabal\logs\data-default-instances-old-locale-0.0.1.log ): Building data-default-instances-old-locale-0.0.1... Preprocessing library data-default-instances-old-locale-0.0.1... [1 of 1] Compiling Data.Default.Instances.OldLocale ( Data\Default\Instances\OldLocale.hs, dist\build\Data\Default\Instances\OldLocale.o ) C:\Programs\Haskell\MinGHC-7.10.1\ghc-7.10.1\mingw\bin\ar.exe: dist\build\libHSdata-default-instances-old-locale-0.0.1-6jcjjaR25tK4x3nJhHHjFM.a-2192\libHSdata-default-instances-old-locale-0.0.1-6jcjjaR25tK4x3nJhHHjFM.a: No such file or directory cabal: Error: some packages failed to install: data-default-instances-old-locale-0.0.1 failed during the building phase. The exception was: ExitFailure 1 Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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foldt fix proposal (HaskellWiki)
by Michael V. Antosha 30 May '15

30 May '15

hi ALL, I think I've found a mistake in foldt implementation on the Fold page[1] of the HaskellWiki. foldt :: (a -> a -> a) -> a -> [a] -> a foldt f z [] = z foldt f z [x] = x -- mistake? -- foldt f z [x] = f x z -- fix proposal foldt f z xs = foldt f z (pairs f xs) pairs :: (a -> a -> a) -> [a] -> [a] pairs f (x:y:t) = f x y : pairs f t pairs f t = t Without the fix, foldt just ignores z value in all cases except for the case of empty input list. (Which is an inconsistent behaviour I suppose.) The Examples page[2] also needs corresponding fix. Is this mail list a right place to ask for the HaskellWiki page fix? Thanks! Michael. [1] Tree-like folds @ Fold @ HaskellWiki https://wiki.haskell.org/Fold#Tree-like_folds [2] Examples @ Fold @ HaskellWiki https://wiki.haskell.org/Fold#Examples -- Michael V. Antosha http://mivael.in.ua xmpp:m@mivael.in.ua (Jabber ID)

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